Posted by erika on Saturday, February 4, 2012 at 11:24am.
speed of boat in still water --- x mph
speed of current --- y mph
distance up river = time (rate) = 3(x-y) = 9
3x-3y = 9
x-y=3
distance down river = 2(x+y) = 9
2x+2y=9
multiply first by 2
2x-2y = 6
2x+2y = 9
add them
4x = 15
x = 15/4 = 3.75 mph
then in x-y = 3
3.75-y = 3
y = .75 mph
the boat has a speed of 3.75 mph in still water, and
the current has a speed of .75 mph
check:
speed with the current = 4.5
distance = 2(4.5) = 9
speed against current = 3
distance = 3(3) = 9
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